1. Field of the Invention
This invention generally relates to the field of a disk drive to be used with the perpendicular magnetic recording method, and more particularly, to control of read/write gate signals for regulating the timing of a read/write operation.
2. Description of the Related Art
In recent years, the perpendicular magnetic recording method has been attracting attention as technique for overpassing the recording density limit of the conventional longitudinal magnetic recording method in the technological field of disk drives including hard disk drives. Perpendicular magnetic recording method can realize a high surface recording density because it provides a relatively high signal resolution and small signal amplitude attenuation for a high linear recording density.
As is the same basic structure of the longitudinal magnetic recording method, a magnetic head unit having a read head and a write head mounted separately on a slider is used in a disk drive that is adapted to the perpendicular magnetic recording method. The read head normally comprises a GMR (giant magnetoresistive) element, whereas the write head is typically an SPT (single pole type) head. When such a magnetic head is designed to be used with the perpendicular magnetic recording method, the read head and the write head are separated from each other by a large gap greater than its counterpart of the longitudinal magnetic recording method in the circumferential direction of the disk. More specifically, the gap separating the read head and the write head of a magnetic head unit to be used with the perpendicular magnetic recording method is about 7 to 8 μm. On the other hand, the gap separating the read head and the write head of a magnetic head unit to be used with the longitudinal magnetic recording method is about 3 to 4 μm.
With a disk drive using such a separate type magnetic head unit, the timing of reading operation of the read head and that of writing operation of the write head have to be regulated relative to each other. Now, this will be described in greater detail below.
FIG. 7 of the accompanying drawing is a schematic block diagram of a conventional disk drive, illustrating a principal part thereof. Referring to FIG. 7, the disk drive comprises a separate type magnetic head unit 2, a preamp circuit 7, a read/write (R/W) channel 8, a disk controller (HDC) 9 and a microprocessor (CPU) 10.
The preamp circuit 7 has a read amp for amplifying the read signal output from the read head of the magnetic head unit 2 and a write amp for converting the write data signal into a write electric signal. The R/W channel 8 is a signal processing IC adapted to process a read/write data signal (including a servo data signal) and has a function of generating a servo sector pulse SSP out of a servo data signal as will be described hereinafter.
The HDC 9 operates as interface for the drive and the host system (not shown) and has a read/write gate generator 90 for generating read/write signals (RG, WG). The CPU 10 is a main control unit for the drive and controls the read/write gate generator 90.
The disk provided in the disk drive has a number of data regions referred to as data tracks and arranged radially on the surface. Each data track has a data format as shown in FIG. 5B of the accompanying drawing. Referring to FIG. 5B, the data format provides servo areas 50 arranged circumferentially at regular intervals and data areas 51 each of which is arranged between two adjacent servo areas. A servo data signal is recorded in each servo area 50 in the manufacturing process of the drive by a dedicated device referred to as a servo track writer. The servo area 50 is a region which is prohibited the operation of recording data during the normal operation of reading and writing data. The servo data signal provides servo information that is used (by the CPU 10) to control the positioning of the magnetic head unit 2.
On the other hand, each data area 51 is divided into a number of data sectors where user data (DATA) are recorded on it. Additionally, note that the data area 51 has a length that is not necessarily equal to the length of a data sector multiplied by an integer. A data sector may be divided into two sectors with a servo area 50 interposed between them.
Each data sector has a gap 52, a PLL synchronizing signal (PLL) 53, a sync byte (SB) 54, a user data (DATA) 55, EC (error correction) related information 56 and a pad 57 as shown in FIG. 6.
Referring still to FIG. 6, gap 52 is a region for absorbing fluctuations in the rotational movement of the disk. The gap 52 is recorded by the write head during the operation of writing data in the data area 51. Then, a PLL synchronizing signal (PLL) 53 is recorded immediately after the recording of the gap 52. The PLL 53 provides a synchronizing signal pattern for synchronizing the read clock to be used for a read operation (data reproducing operation) with the data that is recorded there. Then, a synchronizing byte (sync byte SB) 54 for detecting the starting point of user data is recorded. Subsequently, user data (DATA) is recorded.
Referring now to FIG. 8C, a read operation is executed as the HDC 9 activates the read gate signal RG for the R/W channel 8. The read/write gate generator 90 comprises a data sector pulse (DSP) generator 91 and a delay circuit 92. The read/write generator 90 generates a read gate signal RG and a write gate signal WG by referring to the DSP generated by the DSP generator 91 as shown in FIG. 8B. The DSP generator 91 generates a data sector pulse (DSP) by referring to the servo sector pulse (SSP) output from the R/W channel 8 as shown in FIG. 8A. The servo sector pulse (SSP) is generated at the timing of the servo gate signal (SG) output from the HDC 9.
In the write operation, the write gate signal WG is activated by the data sector pulse (DSP) as shown in FIG. 8D. Immediately after the servo area 50, the data sector pulse (DSP) is generated with a delay equal to the time period corresponding to the gap (GN) between the read/write heads, starting from the timing of the end of the servo area 50 (see FIG. 8E). The delay is necessary because, when the end of the servo area 50 is detected by referring to the read head, the write head is still located within the servo area 50 and therefore the servo data signal recorded in the servo area 50 will be destroyed if any data is written at that time. The delay is provided to avoid this problem.
In the read operation, on the other hand, the read gate signal RG needs to be activated at the middle of the rotary movement fluctuations absorbing gap 52 (the boundary of the parts PA1 and PA2) (see FIG. 8C). Therefore, the delay circuit 92 activates the read gate signal RG at the timing obtained by a delay of a predetermined time period DT from the data sector pulse (DSP). The delay time DT is defined by the CPU 10. As a result, the read head can reliably read the PLL synchronizing signal (PLL) 53 if the rotary movement of the disk fluctuates in the read/write operation. Note that a PLL synchronizing signal is recorded in the rotary movement fluctuations absorbing gap 52. In other words, the PLL region 53 is an area for securing a necessary minimal PLL synchronizing signal.
With the longitudinal magnetic recording method, the gap separating the read/write heads is about 3 μm as pointed out earlier. If the disk rotates at a rate of 4,200 rpm±0.2%, the time period (GN) corresponding to the gap varies depending on the position of the head unit 2 on the disk. More specifically, the time period (GN) is about 0.32 μs on the outer periphery of the disk and about 0.64 μs on the inner periphery of the disk. Therefore, normally, the CPU 10 defines a delay time (GN) for each zone on the disk and gives it to the DSP generator 91. The zones of the disk are produced by dividing the total number of tracks on the disk by a given number.
On the other hand, each rotary movement fluctuations absorbing gap 52 (PA1 and PA2) has a constant length regardless of the position on the disk. If the accuracy of rotary movement is ±0.2%, the time period corresponding to the rotary movement fluctuations absorbing gap 52 is about 1.27 μs. Then, the delay time DT of the read gate signal RG is “(1.27/2−0.32)=0.315 μs” on the outer periphery and substantially equal to 0 on the inner periphery.
As pointed out above, the magnetic head unit 2 of the perpendicular magnetic recording method has a gap between the read/write heads greater than its counterpart of the longitudinal magnetic recording method. Therefore, the timing of generating the data sector pulse (DSP) needs to be relatively delayed as shown in FIG. 9A. If the delay time is reduced to nil, the timing of activating the read gate signal RG the two parts (PA1 and PA2) of the rotary movement fluctuations absorbing gap 52 as shown in FIG. 9B. Under this condition, there may be cases where the PLL synchronizing signal is not properly read out from the PLL region 53 by the read head at the timing of activating the read gate signal RG because of fluctuations in the rotary movement of the disk.
Therefore, it is necessary to absorb fluctuations in the rotary movement of the disk by increasing the region of the rotary movement fluctuations absorbing gap 52 (by PA3 for the PA1) as shown in FIG. 9E. However, the efficiency of the data format is reduced as the region of the rotary movement fluctuations absorbing gap 52 that is not the data recording region of the disk increases.
Now, how the efficiency of the data format is reduced will be described more specifically.
Assume that the gap separating the read/write heads is 8 μm and the disk is driven to rotate at a rate of 4,200 rpm±0.2%. The time period (GN) that corresponds to the gap varies depending on the position of the head unit 2. To be accurate, the time period is about 0.86 μs on the outer periphery and about 1.71 μs on the inner periphery of the disk.
As for the length of the pairs of rotary movement fluctuations absorbing gap 52, the PA1 needs to be increased by a part (PA3) that corresponds to 0.225 μs on the outer periphery and 1.075 μs on the inner periphery, provided that the accuracy of rotary movement is ±0.2% and the delay time DT of the read gate signal is 0. These time periods correspond respectively to 8.4 bytes and 20.1 bytes in terms of a disk drive having a transfer rate of 300 Mbps. In other words, the rotary movement fluctuations absorbing gap 52 has a length equal to 1.3% to 3.2% of a data sector on the disk. Therefore, the efficiency of the data format is reduced by these percentage figures.
Disk drives adapted to delay the operation of recording the data to be written by a time period corresponding to the gap (distance) separating the read/write heads so that the read head may properly reproduce the recorded data (see, inter alia, Jpn. Pat. Appln. KOKAI Publication No. 6-176486 and U.S. Pat. No. 5,600,501). However, the problem of reduced efficiency of the data format cannot be resolved simply by delaying the operation of recording the data to be written on the disk.